Los Angeles — Some $2.4 billion has been spent since 2005 on a still-unfinished project to erect more than 600 miles of new fence along the US-Mexico border – a finding that is being met with surprise, anger, and consternation by immigrant groups and at least some border residents.

A report, released Thursday by the Government Accountability Office (GAO), also says $6.5 billion will be needed to maintain the new fencing over the next 20 years. So far, it has been breached 3,363 times, requiring $1,300 for the average repair.

The US Border Patrol, for its part, agrees with some findings but says several conclusions are unknowable because building the wall has no precedent. And the agency defends the new fencing as effective at deterring illegal immigration.

"When our nation is in the midst of an economic crisis, we wonder how many teacher salaries, police officers, miles of road, or school books could be financed instead of throwing large amounts of money for bricks to fix a problem that requires serious, long-term solutions," says Angelica Salas of the Coalition for Humane Immigrant Rights of Los Angeles, in a statement.

Dawn Garner, who lives on a ranch on the US-Mexico border in Naco, Ariz., says spending is so high because workers who are building the fence use local hotels for accommodations and food.

"They should live in tents near the wall and cook their own food, and that would save incredible amounts of money," says Ms. Garner, who reports that 40 illegal immigrants a day cross her small ranch. Money could be saved if the National Guard built the fence and if the Border Patrol itself maintained it, she suggests in a phone interview.

Despite the price tag of maintaining the border fence, authorities have not found a way to determine whether it is helping to halt illegal immigration, the GAO report says.

"While they [the GAO findings] have highlighted some risks and their factual statements are correct, we are not as pessimistic as they are," says Mark Borkowski, executive director of the Secure Border Initiative, part of US Customs and Border Protection. Trying to analyze a new endeavor like this fence is like trying to calculate the costs and benefits of planes in combat while they're still on the drawing board, he says.

He acknowledges that attempts to assess the efficacy of the new fence are sketchy. The Naco area where Garner lives may be more porous than other parts of Arizona, such as Yuma or Sasabe.

Still, he says, "it is very clear to the Border Patrol that this has been very effective in cutting down illegal migrant traffic into the US."

The 600 additional miles of fence, started under the Bush administration, have seen several delays and cost increases, which Borkowski says are to be expected in such a massive construction project.

Until the various types of border barriers are in place, states the GAO report, the Border Patrol will not know if the added security measures are working.

"The existing border fortifications do not keep undocumented migrants out of the US. Not even half are being apprehended on any given trip to the border, and of those who are apprehended, the success rate on the second or third try is upwards of 95 percent."

"There is no reason to believe that additional investments in the fence project – both physical fencing and the new "virtual fence" – will create an effective deterrent," he says.